Because they stuck together.
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No it wasn't. Some species may have been herd animals but I don't think there's any evidence for that. They were successful because conditions for them were reasonable stable for a very long time. Individual species came and went, of course.
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fish then amphibians then reptiles.
reptiles were more highly evolved then their amphibian ancestors so they took over and grew huge.
because they treated the females as dirty wenches and they all took turns having sex with them creating more and more dinosaurs